US airline worker steals, crashes plane

Federal authorities are searching for what drove an airline worker to steal an empty plane from Seattle airport and crash it into a sparsely populated island, sparking a security scare that saw fighter jets scrambled.

The Horizon Air ground service agent took the controls of a Bombardier Q400 turboprop airplane in a maintenance area at Seattle-Tacoma International Airport about 8 pm on Friday.

He took off, embarking on a flight over Seattle before crashing about an hour later on Ketron Island in Puget Sound, Horizon sister carrier Alaska Airline said online.

The 29-year-old, who has not been identified, was suicidal and appeared to have acted alone, Pierce County Sheriff's Department said on Twitter, adding the employee was probably killed in the crash.

"Doing stunts in air or lack of flying skills caused crash into Island," the sheriff said on Twitter.

The Federal Aviation Administration, the Federal Bureau of Investigation and the National Transportation Safety Board were investigating the incident, said Brad Tilden, the CEO of Alaska Air Group.

"We're working to find out everything we possibly can about what happened," he said.

There was no indication the incident was an act of terror and no passengers were aboard the plane, the FBI said in a series of Tweets.

"The FBI continues to work with our state, local and federal partners to gather a complete picture of what transpired with tonight's unauthorised Horizon aircraft takeoff and crash," the agency said.

It is unclear how the employee was able to taxi the plane on a runway and take off without authorisation. An Alaska Air spokesman said he did not have information about how the incident occurred.

"Okay this insane. A pilot on the plane in front of us just went rogue and took off on an empty plane bypassing orders from the tower," tweeted Ben Schaechter, a passenger on an airplane that was taxiing to take off before the incident.

Video footage on social media showed a large plane flying above the Seattle area with a F-15 fighter jet following it. Two F-15s pursued the plane but were not involved in the crash, the sheriff said.

"Told F-15s made it within a few minutes of theft of plane. Pilots kept plane out of harm's way and people on ground safe," the sheriff said on Twitter.

Alaska Air said the F-15s were scrambled from a Portland air base.

The sheriff said earlier the employee was a airline mechanic but the airline said he was a ground service agent.

Fire crews were working on putting out the fire from the crash on Ketron Island in Puget Sound. No one was injured on the ground, authorities said.